Indicator tube with leads extending through mesa to support cathodes

ABSTRACT

This application discloses an indicator tube which includes, in a gas-filled envelope, a stack of cathodes having upper and lower leads which are sealed in unitary bodies of insulating material, and an anode assembly which includes a back plate behind the cathodes and an optical screen in front of the cathodes. The upper body of insulating material which secures the upper cathode leads is rigidly secured between the anode back plate and screen. Also disclosed is a method of making an indicator tube electrode assembly which comprises stacking cathode electrodes in a mold upside down, the cathodes having upper and lower leads. The upper leads, which face down, are positioned in a container filled with powdered glass and placed at the bottom of the mold. The lower leads, which face up, are threaded on a preformed glass disk which ultimately forms the base of the tube envelope. The entire assembly is heated to cause the glass powder and the glass disk to melt and form a strong, rigid mount for the cathodes through their upper and lower leads, when the assembly is allowed to cool. In a modification of the tube structure, which is particularly adapted for use with the above-described assembly method, the lower glass disk is replaced by powdered glass held in a suitable container in which it is melted and then solidified to form the stem of the tube.

United States Patent [72] Inventor James Thomas Boyer Plainiield, NJ.

[21] Appl. No. 22,131

[22] Filed Apr. 1, 1970 [45] Patented Dec. 21,1971

[7 3] Assignee Burroughs Corporation Detroit, Mich.

Continuation of application Ser. No. 631,109, Mar. 15, 1967, now abandoned. This application Apr. 1, 1970, Ser. No. 22,131

[54] INDICATOR TUBE WITH LEADS EXTENDING THROUGH MESA TO SUPPORT CATHODES Primary Examiner- Robert Segal Attorney-Robert A. Green ABSTRACT: This application discloses an indicator tube which includes, in a gas-filled envelope, a stack of cathodes having upper and lower leads which are sealed in unitary bodies of insulating material, and an anode assembly which includes a back plate behind the cathodes and an optical screen in front of the cathodes. The upper body of insulating material which secures the upper cathode leads is rigidly secured between the anode back plate and screen.

Also disclosed is a method of making an indicator tube electrode assembly which comprises stacking cathode electrodes in a mold upside down, the cathodes having upper and lower leads. The upper leads, which face down, are positioned in a container filled with powdered glass and placed at the bottom of the mold. The lower leads, which face up, are threaded on a preformed glass disk which ultimately forms the base of the tube envelope. The entire assembly is heated to cause the glass powder and the glass disk to melt and form a strong, rigid mount for the cathodes through their upper and lower leads, when the assembly is allowed to cool.

In a modification of the tube structure, which is particularly adapted for use with the above-described assembly method, the lower glass disk is replaced by powdered glass held in a suitable container in which it is melted and then solidified to form the stem of the tube.

INDICATOR TUBE WITH LEADS EXTENDING THROUGH MESA TO SUPPORT CATIIODES This application is a Continuation of application, Ser. No. 631,109, filed Mar. 15, 1967, now abandoned.

BACKGROUND OF THE INVENTION IN INDICATOR TUBE This invention relates to cold cathode gaseous glow tubes and, particularly, to cathode glow indicator tubes for displaying characters such as numbers, letters, or the like.

One type of indicator tube which has been available commercially for many years is known as a NIXIE tube and is made and sold by Burroughs Corporation. The most popular commercial form of this tube includes a stack of cathode electrodes relatively loosely mounted parallel to each other on two support posts in a metallic cup-shaped anode electrode and facing a viewing window at the end of the tube envelope. This type of tube is thus known as an end-view tube. The cathode electrodes are oriented parallel to the base of the tube in which the tube pins are mounted, and the cathodes are shielded from the tube pins by the anode cup and by an assembly of insulating disks. This tube thus includes a relatively large number of components, which are provided primarily for the purpose of insulating and supporting the electrode structure and for shielding the cathode leads and tube pins from other components to prevent these leads and pins from glowing. These components represent a cost factor in both material and assembly time which it would be desirable to eliminate, if possible. In mass production operation, such savings can be important. Similar indicator tubes which have appeared on the market are also subject to the same criticism.

Cathode glow indicator tubes are also constructed as side view tubes, with a stack of electrodes mounted in the tube envelope on two support posts and oriented perpendicular to the tube base and facing the side of the tube envelope. Tubes of this construction present somewhat greater problems than the end-view tube in providing a rugged mounting arrangement for the electrodes within the tube. Thus, these tubes also require a relatively large number of auxiliary elements to provide a satisfactory structure.

The objects of the present invention concern the provision of an improved multiple cathode indicator tube which is of a simple but rugged construction.

The objects of the invention also relate to the provision of a multiple cathode side view indicator tube.

Briefly, an indicator tube embodying the invention includes a gas-filled envelope which is generally elongated and has at one end a dome, at the other end a pin-carrying stem, and in between, a viewing window. The stem carries a plurality of tube pins which extend across a diameter of the stem.

An electrode assembly mounted inside the envelope includes a rear anode plate mounted remote from the viewing window and a front anode screen, both being generally parallel to the longitudinal axis of the envelope. A plurality of cathode glow electrodes are arranged in a stack facing the viewing window and positioned between the front screen and rear anode plate. The cathode electrodes are shaped as characters such as letters, numbers, or the like. The entire electrode assembly is rigidly and directly mounted on the stem with the cathodes having their lower ends supported by leads embedded directly in the stem. The upper ends of the cathodes are also embedded in an insulating body which is rigidly supported between the front screen and rear anode plate.

DESCRIPTION OF THE DRAWING In the drawing:

FIG. I is an isometric view of an indicator tube embodying the invention;

FIG. 2 isa plan view of a portion of the tube of FIG. 1;

FIG. 3 is anelevational view of a glow cathode used in the tube of FIG. 1

FIG. 4 is a side elevational view of apparatus used in assembling the tube of FIG. I and portions thereof;

FIG. 5 is a sectional view of a modification of a portion of the invention; and

FIG. 6 is a sectional view of a modification of another portion of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, an indicator tube 10 embodying the invention includes an envelope 20' which contains a gas suitable for supporting cathode glow. Such a gas may be argon, neon, or the like at a pressure which may be in the range of about 30 to about mm. of mercury. If desired, other substances such as mercury may be added in small quantities to the gas content of the envelope.

The envelope 20 is generally elongated and its sidewall 22 comprises a viewing window so that the tube is what may be called a side view indicator tube. The envelope includes a base or stem 24 through which metal pins or leads 28 extend, and by means of which electrical connection is made from the electrodes within the tube to external electrical circuitry.

The dome or upper end 30 of the envelope carries a tubulation 32 (shown sealed off) for evacuating the envelope and filling it with the necessary gaseous atmosphere.

An insulating disk 26 of mica or the like is seated on stem 24 and a similar mica disk 26' is at the top of the electrode assembly of the tube. One or both of these mica disks may be omitted as those skilled in the art will understand.

A plurality of the tube pins 28 are positioned on a common diameter of the stem, and, to make optimum use of this diameter length, the pins are staggered (FIG. 2) so that, in effect, there are two rows of aligned pins, with adjacent pins in each row being offset from each other. Two other pins 28 are also provided oriented at the end of a second diameter which is generally perpendicular to the first-mentioned diameter.

An electrode assembly mounted inside the envelope includes a rear anode plate 32 mounted remote from the viewing window and a front anode screen 38, both being generally parallel to the tube pins and to the longitudinal axis of the envelope. The back plate 32 and front screen 38 are generally trough shaped, and they are oriented facing each other so that they enclose a generally cylindrical space in which cathode glow electrodes 42 are mounted. The back plate and front screen may have their lower edges resting on the mica disk 26 or on the stem 24, or their lower edges may be provided with short projections 48 which rest on the disk. The screen and plate are electrically connected to the first and last tube pins 28, respectively, and to the other pins 28' to provide added rigidity.

The cathode glow electrodes 42 are arranged in a stack facing the viewing window and positioned between the from screen and rear anode plate. The main body of each cathode electrode is shaped as a character such as a letter, number, or the like, and upper and lower leads 46 and 48, respectively, extend from suitable locations on each cathode to permit proper mounting on the stem, as will be described. The lower lead 48 of each cathode is welded to a tube pin 28 with a relatively short spacing, for example, one-eighth inch being provided between the top surface of the mica disk 26 and the point of attachment of lead 48 to the main body portion of the cathode.

A boxlike metallic shield 58 rests on the disk 26 and encloses the portions of the leads 48 between the character portions of the cathodes and the stem. The shield includes apertures 60 in its top wall 62 which receive and insulate the cathode leads 48 from each other. Thus, the cathode leads are completely shielded, and they either will not glow, or, if they glow, the glow will be masked by the shield and will not be visible. The shield 58 is held in place by means of tabs 59 which are inserted in suitable apertures in the mica disk and are bent over to engage the bottom surface of the disk. Thus, the tabs 59 lie between the disk .26 and the stem 24 and are concealed.

In addition to performing a shielding function, the shield 58 elevates the cathode electrodes 42 above the stem 24 so that there is minimal interference by the stem with viewing of the cathodes.

The upper leads 46 of the cathodes are secured in an insulating block 66 of glass or the like which is enclosed in a metal shield 74 similar to shield 58. The shield 74 is welded at its rear end to the back plate 32 and at its front end to front screen 38 near the upper ends thereof. The upper shield 74 includes suitable apertures 75 through which upper cathode leads 46 pass into glass block 66 without touching the shield. Leads 46 are also staggered if necessary. The above-described interconnections provide a strong electrode assembly.

If the upper mica disk 26' is used, it may rest on the upper edges of the anodes 32 and 38 and it may be held by tabs 77 on the upper edges of the anodes and/or by one or more tabs 79 on the upper shield 74 FIGS). With this arrangement, it may not be necessary to secure shield 74 directly to the rear anode plate 32 and the front anode screen 38.

For optimum operation and long life, the tube includes in its atmosphere a small quantity of mercury vapor which serves to minimize cathode sputtering during tube operation. The mercury may be provided in any suitable fashion. For example, it may be introduced directly with the other gases, such as neon and argon, or, in a preferred arrangement, it is provided in a glass capsule 94 which is surrounded by a coil of heating wire 98 connected between two pins 28. Heating of the wire by current flow causes the mercury to expand and fracture the capsule, whereby the mercury vapor escapes into the tube atmosphere.

One method of practicing the invention utilizes a generally tubular mold 100 (FIG. 4) having openings and formations in its inner wall suitable for receiving the parts to be described. On the floor 103 of the mold is placed a quantity of powdered glass or a block of glass 66 which is enclosed by the shield 74. The cathodes are then positioned in place with their upper leads 46 inserted through the apertures in the shield 74 into the powdered glass or into suitable apertures in the glass block 66, if such is used. The cathodes are held in place in suitable slots formed in the mold and extending from side to side parallel to each other. Next, the'assembly of mica disk 26 and metal shield 58 are set in place with the lower cathode tube leads 28 extending through apertures in the mica disk. Finally, a preformed glass block which becomes stem 24 is set in place on the mica disk, with the leads 28 extending through suitable apertures formed therein. The glass block may comprise the closure for the mold, or, if desired, an auxiliary metal plate (not shown) may be placed on the glass block to close the mold. The mold carrying the aforementioned parts is then heated with high-frequency electrical energy, or it is placed in an oven and suitably heated to cause the glass powder or the glass block 66 and the glass disk 24 to soften and provide intimate engagement with the cathode leads and tube leads, respectively. When the assembly is permitted to cool, and the stem and upper block 66 resolidify, a rigid assembly is provided. The anode screen and rear anode plate are then mounted in place, and the tube is processed to completion in known fashion.

in a modification of the tube described above, the lower metal shield 58 is modifiedto change its mode of attachment to the stern of the tube and to permit the stem to be formed either from a glass block as described above or from powdered glass. Essentially, a modified shield 58 results from enlarging the base of the shield 58 (FIG. 5) to provide (1) a plate 200 covering the top surface of the stem 24 or (2) adding a circumferential lip 210 to the plate 290 to provide a cup-shaped member to receive the stem or (3) adding to (2) an outer plate 220 covering the outer lower surface of the stem and thus to provide a complete container for the stem. Of course, if plate 220 is provided, it would include suitable openings 230 to permit tube pins 28 to pass through. With shield 58', mica disk 26.

may be omitted.

This modified construction permits the stem 24 to he formed of powdered glass, rather than a glass block, if desired. Assuming that mold 100 is used for assembly as above and'the tube parts are mounted upside down, then the modified shield member 58' can act as a support for powdered glass or, without plate 220, it can support a glass block. The metal shield 58' serves as a good heat conductor in the assembly operation, as does the other shield 74.

The tube of the invention has many advantages including that of being readily adapted to automatic assembly because of the minimum number of parts employed and because of their physical arrangement. The tube also has the advantage of providing a strong and rugged assembly of electrodes, due particularly to the manner of anchoring the cathodes in the stem and in the upper insulating block.

Those skilled in the art will appreciate that changes may be made in the tube described above within the scope of the invention. For example, the upper and lower cathode leads may or may not be staggered and offset from each other, depending on the space requirements imposed on the tube. in addition, proper anode operation does not require the front screen, and this member may be omitted, if desired.

What is claimed is:

l. A side view cathode glow indicator tube comprising an envelope having a stem which is substantially flat over a large portion of its interior surface and a sidewall with a viewing window sealed to said stem and containing an ionizable gas at a pressure capable of sustaining cathode glow,

a plurality of character-shaped cathodes disposed within said envelope and arranged in parallel planes in a stack substantially perpendicular to said stem and facing said viewing window,

an anode surrounding said cathodes in operative relation with each of said cathodes,

a mesa in the form of an elongated ridge extending in substantially a straight line across the interior surface of said stem, said mesa being supported directly by and extending upwardly from said interior surface of said stem, at least the upper surface of said mesa being conductive and having apertures therein,

a plurality of conductive leads extending through said mesa by way of said apertures for supporting said cathodes and providing external electrical terminals, said leads extending substantially straight from the upper surface of said mesa into contact with the bottom ends of said charactershaped cathodes to form columnar support posts for said cathodes and extending substantially straight in the opposite direction to exit from the exterior surface of said stem along a path extending across said stern beneath said mesa, said leads, further, being disposed in two separate rows along a substantially straight line path extending across said stem, with the leads of successive cathodes of the stack being offset from one another and located in different ones of the two separate rows but the leads of alternate cathodes being in the same row,

said cathodes being elevated a predetermined distance above the flat portion of the stem, and said mesa extending upwardly from the stern substantially said predetermined distance os that said cathode leads are substantially completely concealed in said mesa,

an upper support member positioned adjacent the upper ends of said cathodes, said support member containing a plurality of apertures along a path substantially parallel to said mesa, and

tablike members connected to the upper ends of said character-shaped cathodes, each extending upwardly from one of said cathodes into one of the apertures in said .cathodes.

4. A side.view cathode glow indicator tube comprising an envelope having a stem which is substantially flat over its interior surface and a sidewall with a viewing window sealed to said stem and containing an ionizable gas at a pressure capable of sustaining cathode glow,

a plurality of character-shaped cathodes disposed within said envelope and arranged in parallel planes in a stack substantially perpendicular to said stem and facing said viewing window,

an anode surrounding said cathodes in operative relation with each of said cathodes,

a conductive mesa in the form of an elongated inverted troughlike member extending across the interior surface of said stem, said mesa being supported directly by and extending upwardly from said interior surface and containing a plurality of apertures apaced apart along its length,

a plurality of conductive leads extending through said apertures for supporting said cathodes and providing external electrical terminals, said leads extending substantially straight from the upper surface of said mesa into contact with the bottom ends of said character-shaped cathodes to form columnar support posts for said cathodes and extending substantially straight in the opposite direction to exit from the exterior surface of said stem beneath said mesa,

said cathodes being elevated a predetermined distance above the interior surface of the stem, and said mesa extending upwardly from the stern substantially said predetermined distance so-that said cathode leads are substantially completely enclosed by said mesa,

an upper support member positioned adjacent the upper ends of said cathodes, said support member containing a plurality of apertures along a path substantially parallel to said mesa, and

tablike members connected to the upper ends of said character-shaped cathodes and extending upwardly from each of said cathodes into one of the apertures in said upper support member.

5. An indicator tube as in claim 4 wherein said upper support member comprises insulating material within a second conductive troughlike member, said second conductive member containing a plurality of apertures through which the respective tablike members of the cathodes extend in closely spaced relation with the edges of said apertures.

6. An indicator tube as in claim 5 wherein at least one of said troughlike members contains an elongated block of insulating material with a plurality of apertures spaced apart along its length.

7. An indicator tube as in claim 5 wherein the conductive leads extending from the bottom ends of said cathodes are tablike members, and wherein at least one of said troughlike members contains solidified powdered glass engaging and supporting the tablike members connected to one end of said cathodes.

8. An indicator tube as in claim 4 wherein the conductive leads and the apertures through said conductive mesa are disposed in two separate rows, with the leads of successive cathodes of the stack being offset from one another and located in different ones of the two separate rows but the leads of alternate cathodes being in the same row.

9. The tube defined in claim 4 and including a disk of insulating material seated on said stem and having said mesa secured thereto and held in place thereby.

10. A side view cathode glow indicator tube including an envelope filled with a gas for sustaining cathode glow,

said envelope including a tubular portion having a dome at its upper end, a sidewall which comprises a viewing window, and a base comprising the closed lower end of said envelope, said envelope having a long vertical axis and said base having a horizontal axis,

said base comprising a body of insulating material generally in the form of a disk and having a generally flat top surface inside the envelope and a bottom surface outside the envelope and having a plurality of electrode contact leads sealed in the base and extending therethrough,

generally cylindrical anode electrode comprising a generally solid rear anode plate and a front anode screen facing each other and defining a generally cylindrical space between them, the axis of the cylinder defined by said plate and screen being parallel to said vertical axis of said envelope,

said anode plate and screen being electrically connected to at least one of said leads,

a plurality of cathode glow electrodes positioned parallel to each other facing said viewing window in said cylindrical space between said anode plate and screen, said cathodes being oriented in parallel planes which are perpendicular to the plane defined by said top surface of said base of said envelope,

each cathode electrode having a main body portion to be viewed and a cathode mounting tab at its lower end adjacent to said base,

each said cathode mounting tab being secured to one of said contact leads, and

a three-dimensional boxlike, conductive, shield seated on said top surface of said base and having a long axis extending between said anode screen and plate, said shield being positioned completely between said main body portions of said cathodes and said top surface of said base and having a top surface having apertures through which said cathode mounting tabs extend with said main body portions of said cathodes having their lower ends positioned closely adjacent to said top surface, said cathode mounting tabs each being secured to its contact lead inside said boxlike shield below said top surface of said box whereby, in effect, said mounting tabs and said contact leads cannot be seen by a viewer.

11. The tube defined in claim 10 and including a second shield member supported by said anode electrode and receiving and supporting the upper ends of said cathode electrodes.

12. The tube defined in claim 10 wherein said cathode electrodes include upper cathode mounting tabs at their upper ends and including an upper insulating block carried by said anode electrode and receiving and supporting said upper cathode tabs.

t t i 

1. A side view cathode glow indicator tube comprising an envelope having a stem which is substantially flat over a large portion of its interior surface and a sidewall with a viewing window sealed to said stem and containing an ionizable gas at a pressure capable of sustaining cathode glow, a plurality of character-shaped cathodes disposed within said envelope and arranged in parallel planes in a stack substantially perpendicular to said stem and facing said viewing window, an anode surrounding said cathodes in operative relation with each of said cathodes, a mesa in the form of an elongated ridge extending in substantially a straight line across thE interior surface of said stem, said mesa being supported directly by and extending upwardly from said interior surface of said stem, at least the upper surface of said mesa being conductive and having apertures therein, a plurality of conductive leads extending through said mesa by way of said apertures for supporting said cathodes and providing external electrical terminals, said leads extending substantially straight from the upper surface of said mesa into contact with the bottom ends of said character-shaped cathodes to form columnar support posts for said cathodes and extending substantially straight in the opposite direction to exit from the exterior surface of said stem along a path extending across said stem beneath said mesa, said leads, further, being disposed in two separate rows along a substantially straight line path extending across said stem, with the leads of successive cathodes of the stack being offset from one another and located in different ones of the two separate rows but the leads of alternate cathodes being in the same row, said cathodes being elevated a predetermined distance above the flat portion of the stem, and said mesa extending upwardly from the stem substantially said predetermined distance os that said cathode leads are substantially completely concealed in said mesa, an upper support member positioned adjacent the upper ends of said cathodes, said support member containing a plurality of apertures along a path substantially parallel to said mesa, and tablike members connected to the upper ends of said charactershaped cathodes, each extending upwardly from one of said cathodes into one of the apertures in said upper support member.
 2. An indicator tube as in claim 1 wherein said conductive surface contains a succession of apertures and each of the cathode leads passes through one of said apertures in closely spaced relation with the edges of said aperture.
 3. An indicator tube as in claim 1 wherein said conductive surface operates as an anode in relation to each of said cathodes.
 4. A side view cathode glow indicator tube comprising an envelope having a stem which is substantially flat over its interior surface and a sidewall with a viewing window sealed to said stem and containing an ionizable gas at a pressure capable of sustaining cathode glow, a plurality of character-shaped cathodes disposed within said envelope and arranged in parallel planes in a stack substantially perpendicular to said stem and facing said viewing window, an anode surrounding said cathodes in operative relation with each of said cathodes, a conductive mesa in the form of an elongated inverted troughlike member extending across the interior surface of said stem, said mesa being supported directly by and extending upwardly from said interior surface and containing a plurality of apertures apaced apart along its length, a plurality of conductive leads extending through said apertures for supporting said cathodes and providing external electrical terminals, said leads extending substantially straight from the upper surface of said mesa into contact with the bottom ends of said character-shaped cathodes to form columnar support posts for said cathodes and extending substantially straight in the opposite direction to exit from the exterior surface of said stem beneath said mesa, said cathodes being elevated a predetermined distance above the interior surface of the stem, and said mesa extending upwardly from the stem substantially said predetermined distance so that said cathode leads are substantially completely enclosed by said mesa, an upper support member positioned adjacent the upper ends of said cathodes, said support member containing a plurality of apertures along a path substantially parallel to said mesa, and tablike members connected to the upper ends of said character-shaped cathodes and extending upwardly from each of said cathodes into one of the apertures in said upper support member.
 5. An indicator tube as in claim 4 wherein said upper support member comprises insulating material within a second conductive troughlike member, said second conductive member containing a plurality of apertures through which the respective tablike members of the cathodes extend in closely spaced relation with the edges of said apertures.
 6. An indicator tube as in claim 5 wherein at least one of said troughlike members contains an elongated block of insulating material with a plurality of apertures spaced apart along its length.
 7. An indicator tube as in claim 5 wherein the conductive leads extending from the bottom ends of said cathodes are tablike members, and wherein at least one of said troughlike members contains solidified powdered glass engaging and supporting the tablike members connected to one end of said cathodes.
 8. An indicator tube as in claim 4 wherein the conductive leads and the apertures through said conductive mesa are disposed in two separate rows, with the leads of successive cathodes of the stack being offset from one another and located in different ones of the two separate rows but the leads of alternate cathodes being in the same row.
 9. The tube defined in claim 4 and including a disk of insulating material seated on said stem and having said mesa secured thereto and held in place thereby.
 10. A side view cathode glow indicator tube including an envelope filled with a gas for sustaining cathode glow, said envelope including a tubular portion having a dome at its upper end, a sidewall which comprises a viewing window, and a base comprising the closed lower end of said envelope, said envelope having a long vertical axis and said base having a horizontal axis, said base comprising a body of insulating material generally in the form of a disk and having a generally flat top surface inside the envelope and a bottom surface outside the envelope and having a plurality of electrode contact leads sealed in the base and extending therethrough, a generally cylindrical anode electrode comprising a generally solid rear anode plate and a front anode screen facing each other and defining a generally cylindrical space between them, the axis of the cylinder defined by said plate and screen being parallel to said vertical axis of said envelope, said anode plate and screen being electrically connected to at least one of said leads, a plurality of cathode glow electrodes positioned parallel to each other facing said viewing window in said cylindrical space between said anode plate and screen, said cathodes being oriented in parallel planes which are perpendicular to the plane defined by said top surface of said base of said envelope, each cathode electrode having a main body portion to be viewed and a cathode mounting tab at its lower end adjacent to said base, each said cathode mounting tab being secured to one of said contact leads, and a three-dimensional boxlike, conductive, shield seated on said top surface of said base and having a long axis extending between said anode screen and plate, said shield being positioned completely between said main body portions of said cathodes and said top surface of said base and having a top surface having apertures through which said cathode mounting tabs extend with said main body portions of said cathodes having their lower ends positioned closely adjacent to said top surface, said cathode mounting tabs each being secured to its contact lead inside said boxlike shield below said top surface of said box whereby, in effect, said mounting tabs and said contact leads cannot be seen by a viewer.
 11. The tube defined in claim 10 and including a second shield member supported by said anode electrode and receiving and supporting the upper ends of said cathode electrodes.
 12. The tube defined in claim 10 wherein said cathode electrodes include upper cathode mounting tabs at their upper ends and including an upper insulating block carried by said anode electroDe and receiving and supporting said upper cathode tabs. 